1. Field of the Invention
The present invention relates to the storage of information and particularly to the generation and retention of a voltage commensurate with the position of a movable member. More specifically, this invention relates to a solid-state analog member with the capability of selecting the maximum value of a plurality of input signals respectively commensurate with the positions of movable members. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
There are numerous industrial control applications which require a "tracking" device capable of providing an indication of control system output when the system is driving a plurality of controller elements. Such "tracking devices" are used to provide a reference for the varying analog control signal so that, in cases of system transfer from automatic to a manual mode of operation, or vice versa, such transfer will be "bumpless". In many cases it is also desirable that the tracking device be capable of driving the control system output, in the manual mode of operation, in either of two opposite control directions. When used to drive the control system output, the tracking device must function as a reference for controlled elements which were not in operation at the time of the transfer from the automatic to the manual control mode.
Prior art memory devices for use with industrial controllers to perform the above briefly described tracking function have been both analog and digital devices. In the conventional analog device the tracking function is accomplished electromechanically through the use of a motor, potentiometer and servo amplifier. As is well known in the art, such motor driven potentiometers do not possess the requisite reliability due to wear on the potentiometer wiper arm and due to the possibility of motor failure.
Previously available digital "trackers" have the attribute of being solid-state devices. Such solid-state "trackers" employ an up-down counter that "remembers" digitally the last level of the controller output signal; the stored digital information being converted into an analog output signal through use of conventional digital to analog converters. Digital "tracking" devices possess a number of inherent disadvantages. Thus, by way of example, a loss of electrical power may cause a loss of memory; this possibility necessitating the use of an auxiliary battery to keep the up-down counter powered for a significant period of time. Also, digital tracking devices require a substantial number of components thus rendering the devices comparatively expensive.
It is also to be noted that, in prior art analog memory circuits which have received a plurality of input signals from controlled elements, it has been conventional practice to average the input signals and to "store" the average value. This technique affords the potential for substantial error should there be a failure of any one averaging circuit component.